Amplifiers

Abstract

A feedforward amplifier compensates for distortions introduced by a main amplifying device by comparing the amplified signal with the original unamplified signal and producing a correction signal, which is related to the difference. The correction signal is combined with the main amplified signal at a combiner. The combiner takes the form of a tetrode or pentode valve which acts as a current source and which is designed so as to constitute part of an output transmission line having a predetermined characteristic impedance. By this means the combiner does not adversely affect the main amplified signal and the capacitance inevitably associated with a tetrode or pentode valve is utilized as part of the reactance of the transmission line and does not adversely affect the frequency response of the amplifier.

I claim: 
     
       1. A feedforward electronic amplification system including an amplifier; comparison means, connected between the amplifier's input and output lines downline of the amplifier, for comparing the shape of the control signal downline of said amplifier with the shape of the final output signal, and for providing a difference signal indicative of any amplifier-induced distortion; correction signal generating means, coupled to the comparison means, for forming in dependence upon the difference signal a correction signal which may be used to compensate for the amplifier-induced distortion; and combiner means, connected between the correction signal generating means and the amplifier's output line downline of the connection with the comparison means, for combining the correction signal with the final output signal so as substantially to cancel the amplifier-induced distortion, the combiner forming part of a section of the said output line, and exhibiting the same characteristic impedance as the output line, said combiner including an amplifying device arranged to operate as a variable current source. 
     
     
       2. A system as claimed in claim 1 and wherein said amplifying device is a tetrode valve or pentode valve. 
     
     
       3. A system as claimed in claim 2 and wherein the plate capacitance of the valve constitutes the shunt reactance of the section of the said output line of which it forms a part.